Thermoplastic olefin polymers, such as linear polyethylene, polypropylene, and olefin copolymers, are formed in polymerization reactions in which a monomer is introduced into a reactor with an appropriate catalyst to produce the olefin homopolymer or copolymer. The polymer is withdrawn from the catalyst reactor and may be subjected to appropriate processing steps and then extruded as a thermoplastic mass through an extruder and die mechanism to produce the polymer as a raw material in particulate form, usually as pellets or granules. The polymer particles are ultimately heated and processed in the formation of the desired end products.
Melt flow is the measure of a polymer's ability to flow under certain conditions. It is typically measured as a melt flow index, which is the amount of polymer that flows over a period of time under specified conditions. Typical melt flow units of measurement are g/10 min. Melt flow provides an indication of the polymer resin's processability, such as in extrusion or molding, where it is necessary to soften or melt the polymer resin. Polymer resins produced with a low melt flow may need to be further modified after their initial polymerization to improve their processability. This is typically done through controlled rheology (CR) techniques wherein the molecular weight of the polymer is lowered, usually by the addition of peroxide, to thereby improve its flowability. This secondary processing, however, adds additional processing steps and increases the cost of manufacturing. Controlled rheology processing may also degrade the polymer and leave peroxide residue so that its use may be limited in certain applications. As defined herein, “peroxide residue” shall be construed to mean the decomposition and reaction products of peroxide, such as tert-butyl alcohol, as well as unreacted peroxide, typically found in CR-modified polymers.
Polypropylene is most often produced as a stereospecific polymer. Stereospecific polymers are polymers that have a defined arrangement of molecules in space. Both isotactic and syndiotactic propylene polymers, for example, are stereospecific. Isotactic polypropylene is characterized by having all the pendant methyl groups oriented either above or below the polymer chain. Isotactic polypropylene can be illustrated by the following chemical formula:

This structure provides a highly crystalline polymer molecule. Using the Fisher projection formula, the stereochemical sequence of isotactic polypropylene, as shown by Formula (2), is described as follows:

Another way of describing the structure is through the use of NMR. Bovey's NMR nomenclature for an isotactic pentad is . . . mmmm . . . with each “m” representing a “meso” dyad, or successive methyl groups on the same side of the plane of the polymer chain. As is known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.
Conventional Ziegler-Natta catalysts are stereospecific complexes formed from a transition metal halide and a metal alkyl or hydride and are used in the production of isotactic polyolefins. Ziegler-Natta catalyst for the polymerization of olefins are well known in the art. The Ziegler-Natta catalysts are derived from a halide of a transition metal, such as titanium, chromium or vanadium with a metal hydride and/or metal alkyl, typically an organoaluminum compound as a co-catalyst. The catalyst is usually comprised of a titanium halide supported on a magnesium compound. Ziegler-Natta catalysts, such as titanium tetrachloride (TiCl4) supported on an active magnesium dihalide, such as magnesium dichloride or magnesium dibromide, as disclosed, for example, in U.S. Pat. Nos. 4,298,718 and 4,544,717, both to Mayr et al. are supported catalysts. Silica may also be used as a support. The supported catalyst may be employed in conjunction with a co-catalyst such as an alkylaluminum compound, for example, triethylaluminum (TEAL), trimethyl aluminum (TMA) and triisobutyl aluminum (TIBAL).
Ultra high melt flow (UHMF) polypropylene generally has a melt flow of greater than about 300 g/10 min. The production of UHMF polymers can be achieved during their initial polymerization, without the need for secondary processing. This usually involves the addition of hydrogen during the polymerization reaction. Increasing hydrogen concentrations in the polymerization reactor, however, can result in the production of excessive xylene solubles, which is oftentimes undesirable. Equipment or process limitations may also limit the amount of hydrogen that can be used during the polymerization reaction.
The preparation of ultra high melt flow products during polymerization is a challenge involving a delicate balance between the desired melt flow and xylene solubles. Xylene solubles is a measure of the crystallinity or tacticity of the polymer, or a deviation from the mmmm pentad levels found in isotactic polymers discussed previously. Because increasing hydrogen level generally results in the production of higher xylene solubles, external donors have been used to offset or reduce the amount of xylene solubles levels.
External donors act as stereoselective control agents to control the amount of atactic or non-stereoregular polymer produced during the reaction, thus reducing the amount of xylene solubles. Examples of external donors include the organosilicon compounds such as cyclohexylmethyl dimethoxysilane (CMDS), dicyclopentyl dimethoxysilane (CPDS) and diisopropyl dimethoxysilane (DIDS). External donors, however, tend to reduce catalyst activity and tend to reduce the melt flow of the resulting polymer.
To obtain polymers with the desired high melt flow and reduced xylene solubles, a precise balance between hydrogen concentration and external donors must be struck. Therefore, obtaining ultra high melt flow polymers with low xylene solubles through the use of external donors has been quite difficult, and oftentimes results in significant production of off-grade or unacceptable materials when precise parameters are not maintained.